When students receive vouchers and schools are forced to compete, test scores rise, a new study shows.

Researchers Jay Greene and Marcus Winters of the Manhattan Institute based their study on Florida, which grades its schools each year based on performance on the state's high-stakes FCAT test.

If a school earns two F's within a four-year period, its students win the option of using vouchers to switch to private schools or other public schools.

They found that schools whose students received vouchers made significant gains compared to other schools in Florida.

Schools that had received one failing grade in a three-year period and were at risk of having students receive vouchers also improved, though not as much as their failing counterparts.

Schools that were scoring D's on the test's scale and did not face a voucher situation did not make any significant gains.

The only group of schools that were touched by Florida's voucher program but did not improve were the schools that had received one F more than four years ago and were no longer threatened by vouchers.

They made gains when they were directly at risk of having their students granted the option of using vouchers, but lost ground compared to other Florida public schools after the threat was removed.

Mr. Winters said the findings contradict the leading argument against vouchers: that when students use vouchers to pay for a private or parochial school, they are take away money from the public school and do harm to an already failing institution.

"What this study is telling us is that these programs not only don't hurt the public schools, they're actually causing the public schools to get better," he said. "I think it's clear that vouchers provide a motivation for them to improve because they don't want to lose the money."

He said he hopes politicians notice the study.

"I should hope it has policy implications," he said. "I think it's one more study with consistent findings."

He said in addition to teaching education officials about vouchers specifically, the study could add to debates about school choice, like the ongoing conversation over charter schools in New York State.

He said charter schools might have the "same type of effect" as vouchers by giving families choice and providing incentive for improvement.

The study will be published in the next edition of the education journal, Education Next.